1. Field of Invention
This invention concerns a system for conducting wireline formation testing. More particularly, the invention concerns an improved wireline formation testing method that characterizes mudcake properties and more accurately measures formation characteristics such as compressibility by monitoring fluid seepage through the mudcake prior to any drawdown or buildup sequences. The method of the invention may be especially advantageous for application in supercharged regions.
2. Description of Related Art
Due to the increasing costs associated with drilling oil wells, and due to the increasing availability of "high-tech" well analysis systems, wireline well logging ("wireline logging") has become an important technique to optimize the productivity of oil wells. Generally, in wireline logging, a sensitive measuring instrument is lowered down a wellbore, and measurements are made at different depths of the well. The measuring instrument may take various forms as required, for example, to perform electrical logs, nuclear logs, and formation pressure testing logs. Electrical logs are typically used to locate hydrocarbon reserves. In contrast, nuclear logs are employed to determine the volume of hydrocarbons in the reserves, typically by determining the porosity of the materials in reserves identified by the electrical logs. In contrast to electrical and nuclear logs, formation pressure testing logs ("formation testing logs") are used to determine the mobility of the reserves, chiefly by determining their pressure and permeability.
A wellbore is typically filled with a drilling fluid such as water or a water-based or oil-based drilling fluid. The density of the drilling fluid is usually increased by addingcertain types of solids that are suspended in solution. Drilling fluids containing solids are often referred to as "drilling muds." The solids increase the hydrostaticpressure of the wellbore fluids to help maintain the well and keep fluids of surrounding formations from flowing into the well. Uncontrolled flow of fluids into a well can sometimes result in a well "blowout."
Drilling fluids create a "mudcake" as they flow into a formation by depositing solids on the inner wall of the wellbore. The wall of the wellbore tends to act like a filter. The mudcake helps prevent excessive loss of drilling fluid into the formation. The static pressure in the well bore and the surrounding formation is typically referred to as "hydrostatic pressure." Relative to the hydrostatic pressure in the wellbore, the hydrostatic pressure in the mudcake decreases rapidly with increasing radial distance. Pressure in the formation beyond the mudcake gradually tapers off with increasing radial distance outward from the wellbore.
As shown in FIG. 1, pressure is typically distributed in a wellbore through a formation as shown by the pressure profile 100. Pressure is highest at the wellbore's inner wall, i.e., the inside surface of the mudcake at point 102. The mudcake acts like a filter, restricting the flow of fluids from the high pressure of the wellbore into the relatively lower pressure of the formation. Thus, there is a rapid pressure drop through the mudcake. The pressure at point 104 at the interface between the mudcake and the formation (the "sandface pressure") is substantially lower than the pressure at point 102 at the inside surface of the mudcake. Conventional mudcakes are typically between about 0.25 and 0.5 inch thick, and polymeric mudcakes are often about 0.1 inch thick. Beyond the mudcake, the formation exhibits a gradual pressure decrease illustrated by the slope 106 and asymptotically approaching formation pressure 109. Curve 107 depicts a pressure profile of highly supercharged well with a low permeability mudcake and high sandface pressure 108.
With this type of knowledge, formation testing tools ("formation testers") maybe used to predict the pressure of an oil bearing formation around a well, and to thereby better understand the oil's mobility. In a typical formation testing operation, a formation tester 200 is lowered into a wellbore 202 with a wireline cable 201, as illustrated in FIG. 2A. Inside the wellbore 202, the formation tester 200 resides within drilling fluid 204. The drilling fluid 204 typically forms a layer of mudcake 206 on the wails of the wellbore 202, in accordance with known techniques. In many cases, additioinal logging tools (not shown) for conducting other types of logs, such as gamma ray logs, may be included as part of a tool stiring attached to the same wireline cable and may be located above or below formation tester 200 in the tool string.
After the formation tester 200 is lowered to the desired depth of the wellbore 202, along with any other equipment connected to the wireline cable 201, pressure in a flow line 219 is equalized to the hydrostatic pressure of the wellbore by opening an equalization valve 214. Since the equalization valve 214 is located at a high point of the tester 200, openingthe valve 214 permits bubbles and lighter fluids to escape out into the wellbore 202 through the flow lines 215. Then, a pressure sensor 216 may be used to measure the hydrostatic pressure of the drilling fluid. In the illustrated embodiment, the equalization valve 214 is a two-way valve that simply enables or disables fluid flow through the flow lines 215.
After the equalization valve 214 is again closed, the tester 200 is secured in place by extending hydraulically actuated feet 208 and an opposing isolation pad 210 against opposite sides of the wellbore walls. The pad 210 surrounds a hollow probe 212 (sometimes called a "snorkel"), which is connected to plumbing internal to the tester 200, as described below. Initially, as the pad 210 is extended against the wellbore wall, the pressure inside the probe 2 12 slightly increases.
Fluid from the formation 222 is drawn into the tester 200 by mechanically retracting a pretest piston 218. The retracting of the protest piston 218 creates a pressure drop at the probe 212, thereby drawing formation fluid into the probe 212, the flow lines 219, and a protest chamber 220. The isolation pad 210 helps prevent borehole fluids 204 from flowing outward through the mudcake 206 and circling back into the probe 212 and the chamber 220. Thus, the isolation pad 210 "isolates" the probe 212 from the borehole fluids 204, helping to ensure that the measurements of the probe 212 are representative of the pressure in the formation 222. When the piston 218 stops retracting, formation fluid continues to enter the probe 212 until the pressure differential between the chamber 220 and the formation 222 is minimized.
During the process described above, a number of measurements may be taken. "Drawdown pressure", for example, corresponds to the pressure detected by the sensor 216 while formation fluid is being withdrawn from the formation. In addition, the "buildup pressure" corresponds to the pressure detected while formation fluid pressure is building up again after the drawdown period, i.e., soon after the pretest piston 218 stops moving. Also, the rate at which the piston 218 is retracted may be measured. Furthermore, if further fluid samples are desired in addition to the fluid in the chamber 220, control valves 224 may be individually opened and closed at selected times to capture fluid samples in supplemental chambers 226.
After the desired measurements are made, the formation tester 200 may be raised or lowered to a different depth to take another series of tests. At each depth, the tests usually require a short period of time, such as five minutes. Later, the fluid samples are examined and the measured fluid pressures are analyzed to determine the fluid mobility, as influenced by factors such as the porosity and permeability of adjacent formation.
Normally, the mudcake acts like a filter, largely isolating the high pressure fluids of the wellbore from the relatively lower pressures of the formation. Under these circumstances, the formation pressure tester will detect pressure as shown by the curve 300 illustrated in FIG. 3. Initially, as shown by the portion 301 of the curve 300, pressure at the probe decreases rapidly as the mudcake is sucked into the probe during the "drawdown" period. As shown by the portion 302 of the curve 300, the pressure eventually normalizes (302) as the probe removes fluids from locations that are more and more distant from the wellbore. When the protest piston 218 stops, fluid pressure is allowed to build up again (303), and pressure increases and eventually normalizes to a value corresponding to the formation pressure (304).
Although conventional formation testing systems have been satisfactory in many applications, they are limited when considered for certain measurements. For example, despite the use of the isolation pad 210, during formation testing a significant amount of fluid often flows out into the formation 222 from the wellbore proximate the pad 210, and is thereafter sucked back into the probe 212. This phenomenon is due, at least in part, to the permeability of the mudcake, which allows fluid flow through the mudcake. However, in measuring formation pressure and related parameters, known formation testing techniques fail to compensate for this phenomenon. Therefore, measurements taken with known methods may not be as accurate as some people might require, since they,fail to take into account, the permeability of the mudcake.
Known methods disregard the effect of the mudcake. In one popular technique, for example, the probe is specifically operated to clean away the mudcake to achieve a more effective seal with the formation. This may be performed, for example, by rapidly withdrawing the piston to suck nearby mudcake into the probe, or by extending a pad-cleaning piston (not shown) to perforate the mudcake. In another example, the probe is surrounded by a circular metal ring (not shown) which, in many cases, has the effect of puncturing or entirely removing the mudcake proximate the probe. In this method, the characteristics of the mudcake are clearly not measured, since the mudcake is often effectively removed.
In another technique, two drawdown, cycles are performed--the first cycle establishes a hydraulic seal between the probe and the formation, and the second cycle tests the pressure of the formation. The timing and intensity of suction applied in the first cycle of this method often dislodges or damages the the mudcake near the probe.
Another problem with conventional formation testing systems is that they are not as accurate as some people might desire when used in "supercharged regions." In a supercharged region, the mudcake fails to adequately hold the drilling fluid in the wellbore, and the drilling fluid penetrates the formation creating an "invaded zone." In the invaded zone, the fluid pressure is increased. The effect of supercharging on the operation of a formation pressure tester is illustrated by the curve 305 in FIG. 3. With supercharging, the pressure detected by the formation tester is, initially higher (306) than without supercharging. During drawdown, as the pretest piston 218 retracts, the pressure rapidly decreases (307), but normalizes at a level (308) greater than the non-supercharged formation pressure (302). When the pretest piston 218 stops, fluid pressure rapidly builds up again (309), and pressure increases and eventually normalizes to a value (310) corresponding to the supercharged formation pressure. When the formation pressure testing tool is disengaged from the wellbore, the detected formation pressure rises again (312). This final pressure increase occurs due to the removal of pressure applied by the pad 210.
There are two mechanisms that cause the flow of formation fluid into the probe 212 in the buildup state. First, the compressibility of the fluid in the formation 222 creates a pressure differential between the probe 212 and the formation pressure. The second mechanism is the compressibility of the fluid in the flow line 219 in contact with,the probe 212. This fluid is decompressed, creating an additional pressure differential between the probe 212 and the formation 222. However, many conventional analysis technique ignore these mechanisms, assuming that the wellbore pressure is isolated from the formation near the probe and that little or no fluid flows across the mudcake. As discussed above, fluid how across the Wellbore boundary may be significant due to the, permeability of the mudcake, and such flow may be especially acute in supercharged regions. Therefore, known methods for measuring formation pressure are not as accurate as some people would, like; especially, when applied in supercharged regions.
Some known methods attempt to compensate for the distorting effect of supercharging by measuring formation pressure at various depths and by making estimations based on deviations from a linear pressure relationship. Although this approach might, be adequate for some applications, it is limited because it fails to actually quantify the effect of supercharging, and therefore lacks the level of accuracy some people require.
The present invention is directed to overcoming or minimizing one or more of the problems mentioned above.